Preheat oven to 350°. In a medium bowl, pour 3 cups boiling water over New Mexico chiles and let soak until softened, about 30 minutes. Drain, reserving 1 cup liquid.
Meanwhile, grind pasilla negro chiles, star anise, fennel seed, salt, and peppercorns in a spice mill or clean coffee grinder.
Rub brisket with spice blend; set aside.
Whirl soaked chiles, vinegar, and chile liquid in a blender until puréed. Set aside.
Heat a large roasting pan (not nonstick) over medium-high heat (set it over 2 burners if necessary). Add olive oil, swirl oil in pan, and add brisket. Cook brisket until well browned on one side, about 3 minutes. Turn and brown on remaining sides, adjusting heat to keep brisket sizzling but not burning. Remove and set aside.
Add onion and cook, stirring, until soft, about 3 minutes. Chop tomatoes and add with juice. Add chile purée and broth. Bring to a boil. Add brisket, cover, and bake in oven until fork-tender, 2 to 3 hours.
Place brisket on a serving platter, cover with foil, and set in a warm place. Whirl pan juices in a blender until smooth. Slice brisket thinly and serve hot, with blended pan juices on the side.
Note: Nutritional analysis is per serving.